1. Field of the Invention
The present invention relates to processes for the continuous production of lactide and lactide polymers from crude lactic acid and esters of lactic acid in the field of biodegradable polymers.
2. Description of the Prior Art
The continued depletion of landfill space and the problems associated with incineration of waste have led to the need for development of truly biodegradable polymers to be utilized as substitutes for non-biodegradable or partially biodegradable, petrochemical-based polymers. The use of lactic acid and lactide to manufacture a biodegradable polymer is well known in the medical industry. As disclosed by Nieuwenhuis et al. (U.S. Pat. No. 5,053,485), such polymers have been used for making biodegradable sutures, clamps, bone plates and biologically active controlled release devices. It will be appreciated that processes developed for the manufacture of polymers to be utilized in the medical industry have incorporated techniques which respond to the need for high purity and biocompatibility in the final polymer product. Furthermore, the processes were designed to produce small volumes of high dollar-value products, with less emphasis on manufacturing cost and yield. It is believed that prior to Applicants' development, viable, cost-competitive processes for the continuous manufacture of purified lactide and lactide polymers from lactic acid having physical properties suitable for replacing present petrochemical-based polymers in packaging, paper coating and other non-medical industry applications were unknown.
It is known that lactic acid undergoes a condensation reaction to form polylactic acid when water is removed by evaporation or other means. The overall polymerization reaction is represented by: ##STR1##
While step n of said polymerization reaction is represented by: ##STR2##
As Dorough (U.S. Pat. No. 1,995,970) recognized and disclosed, the resulting polylactic acid is limited to a low molecular weight polymer of limited value, based on physical properties, due to a competing depolymerization reaction in which the cyclic dimer of lactic acid, lactide, is generated. As the polylactic acid chain lengthens, the polymerization reaction rate decelerates until it reaches the rate of the depolymerization reaction, which effectively, limits the molecular weight of the resulting polymers. An example of this equilibrium reaction is represented below. ##STR3##
Given this understanding, Dorough was convinced that high molecular weight polymers could not be generated directly from lactic acid. He was, however, successful in generating high molecular weight polymers from lactide, through the lactic acid dimer generated from the low molecular weight polymers of lactic acid. Because these polymers are generated from lactide, they are known as polylactides.
It is well known that lactic acid exists in two forms which are optical enantiomers, designated as D-lactic acid and L-lactic acid. Either D-lactic acid, L-lactic acid or mixtures thereof may be polymerized to form an intermediate molecular weight polylactic acid which, upon further polymerization, generates lactide as earlier disclosed. The lactide, or the cyclic dimer of lactic acid, may have one of three types of optical activity depending on whether it consists of two L-lactic acid molecules, two D-lactic acid molecules or an L-lactic acid molecule and a D-lactic acid molecule combined to form the dimer. These three dimers are designated L-lactide, D-lactide and meso-lactide, respectively. In addition, a 50/50 mixture of L-lactide and D-lactide with a melting point of about 126.degree. C. is often referred to in the literature as D,L-lactide.
DeVries (U.S. Pat. No. 4,797,468) recently disclosed a process for the manufacture of lactide polymers utilizing a solvent extraction process to purify lactide prior to polymerization. With DeVries' disclosure, the inventor recognized that existing literature recommends purification of lactide by several recrystallization steps. It is believed that processes prior to DeVries solvent extraction method, have generally utilized a recrystallization step to purify the crude lactide in order to obtain a source of lactide suitable for polymerization. However, processes utilizing such recrystallization steps are known to have relatively poor yields due to significant losses of lactide during the recrystallization steps. It is believed that producers of medical-related biodegradable products have not been concerned with such low yields because of the high margin generally expected for sales of such products and the lack of competitive alternatives. It will be appreciated, however, that in developing a process for the large-scale, commercial manufacture of biodegradable polymers, such as polylactides, for use in nonmedical-products-oriented applications where such polymers will necessarily compete with low-cost polymers made from petrochemicals, it will be important to maximize yield and minimize other overall cost factors to produce a biodegradable polymer which is cost-competitive.
The biodegradable polylactide polymers must also possess physical properties suitable for application in non-medical products presently utilizing petrochemical-based polymers such as packaging materials, paper coatings and any other disposable articles. Nieuwenhuis et al. disclose that lactide polymers derived from polymerization of mixtures of the three lactides result in polymers with a variety of useful physical properties, including improved biodegradability. However, no commercially viable process for the large-scale manufacture of such lactide polymers is believed to have been disclosed to date.
Lactic acid is commercially available and manufactured from several known processes. Representative examples of such processes are disclosed by Glassner et al. (European Pat. Application, EP 393818, Oct. 24, 1990), G. Machell, "Production and Applications of Lactic Acid", Industrial Chemist and Chemical Manufacturer, v. 35, pp. 283-90 (1959) and Kirk Othmer, Encyclopedia of Chemical Technology, "Lactic Acid", v. 12, pp. 177-78 (2nd ed. 1963).
The optical activity of either lactic acid or lactide is known to alter under certain conditions, with a tendency toward equilibrium at optical inactivity, where equal amounts of the D and L enantiomers are present. Relative concentrations of D and L in the starting materials, the presence of impurities or catalysts and time at varying temperatures and pressures are known to affect the rate of such racemization.
Muller (U.S. Pat. No. 5,053,522) discloses that the preparation of optically pure lactide from an optically pure lactic acid feed is possible when utilizing appropriate conditions and catalysts. However, it is believed that there are no teachings of processes that control the optical purity of the resulting lactide to desired degrees or minimizes overall costs and maximizes yield of the lactide product. Furthermore, it is believed that there are no disclosures, prior to work described in this and parent applications, of a commercially-viable lactide purification system, which allows production of polymer grade lactide, from crude lactic acid, which may subsequently be polymerized to produce a variety of non-medical-related polylactide polymers suitable for replacing existing petrochemical-based polymers.
Bellis (U.S. Pat. No. 4,727,163) discloses the use of an ester of an alpha-hydroxy acid on a thermally-stable polyether core to manufacture highly pure cyclic esters such as lactide. Bhatia (U.S. Pat. No. 4,835,293) discloses a process for preparing highly pure cyclic esters such as lactide by heating a polymer of the corresponding alpha-hydroxy acid or its ester or a copolymer of the alpha-hydroxy acid or its ester and a thermally-stable polyether core in the presence of an inert gas with the cyclic ester being carried from this reaction with the inert gas to a solvent system. Bellis et al. (PCT Application No. WO 92/00292, published Jan. 9, 1992) disclose a continuous catalyzed vapor phase process for the production of dimeric cyclic esters such as lactide by converting the corresponding alpha-hydroxy carboxylic acid or ester in the vapor phase over a solid catalyst such as silica alumina and preferably silica alumina having a high silica content, in the presence of a carrier gas. However, it is believed that none of these references disclose a commercially viable overall process for the large scale manufacture of polylactide polymers. Furthermore, there is no disclosure, prior to work described in this and parent applications, of a lactide generation system which allows production of a polymer grade lactide for use in non-medical-related polylactide polymers cost-effectively suitable for replacing existing petrochemical-based polymers.